Mechanisms of adjustment to energy source downshift. (1) We have identified a control mechanism in downshifted cells which affects the stability of newly made ribosomal RNA, and responds to some signal other than ppGpp. A possible effector of this control mechanism has also been identified, an unusual gaunine nucleotide which is currently being characterized. (2) We have identified two common elements in the control of ppGpp and cAMP accumulation during downshift. One is the relA gene product, mutation in which leads to overproduction of cAMP. The other is the locus of a new mutation we have isolated and mapped, in which both cAMP and ppGpp production are curtailed. Regulatory nucleotides. We have obtained preliminary evidence for the occurrence of ppGpp in a eukaryotic cell (yeast); and have detected several other novel nucleotides in cultured mammalian cells. The translational defect in relA mutant cells. We have obtained strong evidence for misreading of the genetic code in relA mutants, by demonstrating the illegitimate incorporation of cysteine into a protein which normally lacks that amino acid.